U.S. patent application number 10/868741 was filed with the patent office on 2004-12-30 for low power auto-refresh circuit and method for dynamic random access memories.
Invention is credited to Blodgett, Greg A., Cowles, Timothy B., Shirley, Brian M..
Application Number | 20040268018 10/868741 |
Document ID | / |
Family ID | 22007458 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040268018 |
Kind Code |
A1 |
Cowles, Timothy B. ; et
al. |
December 30, 2004 |
Low power auto-refresh circuit and method for dynamic random access
memories
Abstract
A power saving circuit disables input buffers for command and
address signals during an auto-refresh of a DRAM. The input buffers
are re-enabled at the end of the auto-refresh in a manner that does
not cause spurious commands to be generated. The power saving
circuit prevents spurious commands by biasing internal command
signals to a "no operation" command whenever the input buffers for
the command signals are disabled. The DRAM may also be placed in a
mode in which it automatically transitions to a low power precharge
mode at the end of the auto-refresh to further reduce power
consumed by the DRAM.
Inventors: |
Cowles, Timothy B.; (Boise,
ID) ; Shirley, Brian M.; (Boise, ID) ;
Blodgett, Greg A.; (Nampa, ID) |
Correspondence
Address: |
Edward W. Bulchis, Esq.
DORSEY & WHITNEY LLP
Suite 3400
1420 Fifth Avenue
Seattle
WA
98101
US
|
Family ID: |
22007458 |
Appl. No.: |
10/868741 |
Filed: |
June 14, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10868741 |
Jun 14, 2004 |
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10056935 |
Oct 18, 2001 |
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6771553 |
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Current U.S.
Class: |
711/1 |
Current CPC
Class: |
G11C 11/40611 20130101;
G11C 2211/4067 20130101; G11C 2211/4065 20130101; G11C 11/40615
20130101; G11C 11/406 20130101 |
Class at
Publication: |
711/001 |
International
Class: |
G06F 012/00 |
Claims
1. A power saving circuit for use in a dynamic random access memory
during refresh of the dynamic random access memory ("DRAM"), the
power saving circuit comprising: a first set of input buffers
operable to generate respective internal command signals from
external command signals applied to the input buffers, the input
buffers in the first set being disabled by a first refresh signal;
a bias circuit operable to bias at least one internal command
signal responsive to a second refresh signal; and a refresh decoder
operable to decode at least one internal command indicative of a
refresh of the DRAM and causing the DRAM to be refreshed responsive
thereto, the refresh decoder generating the first and second
refresh signals during the refresh of the DRAM.
2-111. (Cancelled)
Description
TECHNICAL FIELD
[0001] This invention relates to dynamic random access memories,
and, more particularly, to a circuit and method for reducing the
power consumed by such memories when operating in an auto-refresh
mode.
BACKGROUND OF THE INVENTION
[0002] The power consumed by integrated circuits can be a critical
factor in their utility in certain applications. For example, the
power consumed by memory devices used in portable personal
computers greatly affects the length of time they can be used
without the need to recharge batteries powering such computers.
Power consumption can also be important even where memory devices
are not powered by batteries because it may be necessary to limit
the heat generated by the memory devices.
[0003] In general, memory device power consumption increases with
both the capacity and the operating speed of the memory devices.
The power consumed by memory devices is also affected by their
operating mode. A dynamic random access memory ("DRAM"), for
example, will generally consume a relatively large amount of power
when the memory cells of the DRAM are being refreshed because rows
of memory cells in a memory cell array are then being actuated in
the rapid sequence. Each time a row of memory cells is actuated, a
pair of digit lines for each memory cell are switched to
complementary voltages and then equilibrated, thereby consuming a
significant amount power. As the number of columns in the array
increases with increasing memory capacity, the power consumed in
actuating each row increases accordingly. Power consumption also
increases with increases in the rate at which the rows of memory
cells are actuated. Thus, as the operating speed and capacity of
DRAMs continues to increase, so also does the power consumed
increase during refresh of memory cells in such DRAMs.
[0004] During a DRAM refresh, power is also consumed by components
other than those in the memory cell array. For example, DRAM
devices generally include a large number of input buffers to couple
a large number of control and address lines to internal circuitry.
While the DRAM is being refreshed, these input buffers continue to
switch responsive to control and address signals applied to their
respective inputs. However, during some refresh modes, control and
address signals are not used by the DRAM. In an auto-refresh mode,
for example, an auto-refresh command is applied to the DRAM. The
DRAM thereafter internally performs a refresh operation for a
predetermined period of time. During this period, the DRAM does not
respond to control and address signals applied to its input
buffers. However, the input buffers continue to switch during this
time. Switching these large number of input buffers during an
auto-refresh cycle wastes power because, as mentioned above, the
signals coupled through the input buffers are not used during an
auto-refresh cycle.
[0005] In the past, attempts have been made to minimize the power
consumption of DRAMs during auto-refresh by removing power to all
input buffers except input buffers for clock ("CLK") and clock
enable ("CKE") signals. However, leaving the input buffer for the
clock active causes the input buffer to consume a significant
amount of power during the auto-refresh period since the input
buffer toggles with each clock signal transition. Power could be
significantly reduced by removing power to the input buffer for the
clock signal during the auto-refresh period. But doing so could
cause spurious commands to be registered at the conclusion of the
auto-refresh period. As is known in the art, memory commands are
typically registered by latching command signals into respective
latches responsive to one or both edges of the clock signal. If a
clock edge occurs during the time that the input buffers for the
command signals are being re-powered after the auto-refresh period,
a spurious command corresponding to the transitional states of the
input buffers may be registered. Although care can be taken to
avoid coupling clock signal transitions to a memory device until
re-powering of the input buffers have been completed, a spurious
clock signal transition may be generated. A spurious clock signal
transition can be generated if the clock signal has a high logic
level when the input buffer for the clock signal is re-powered. The
spurious clock signal will then register whatever spurious command
corresponds to the logic levels at the outputs of the input buffers
for the command signals.
[0006] In the past, attempts have been made to reduce power during
a self-refresh cycle by removing power from the input buffers
during the self-refresh period. For a self-refresh command,
spurious commands are avoided by first detecting a low-to-high
transition of the CKE signal, which signifies the end of the
self-refresh. However, the input buffers for the command and
address signals are not re-powered at that time. Instead, the
output of a small input buffer coupled to the CLK is examined to
detect a high-to-low transition of the CLK signal. When the
high-to-low transition of the CLK signal is detected, the input
buffers for the command and address signals are re-powered so that
they will not be in a transitional state by the time the next
low-to-high transition of the CLK signal occurs, which is used to
register the commands and addresses.
[0007] Although the approach described above does reduce power
consumption during self-refresh without the risk of registering
spurious commands and addresses, this approach is not suitable for
use during an auto-refresh cycle. Unlike a self-refresh command,
for which the controlling specification allows a delay of two CLK
periods to exit the self-refresh cycle, the controlling
specification for an auto-refresh command requires the DRAM to be
able to register a command occurring on the very next rising edge
of the CLK signal. However, the input buffers for the command and
address signals may still be in a transitional state at that time,
thereby causing spurious command or addresses to be registered.
[0008] One approach to minimizing power consumption during an
auto-refresh cycle is to remove power from some of the command and
address input buffers, but not the input buffers for the clock and
clock enable signals, for a predetermined period after the start of
an auto-refresh cycle. For example, if an auto-refresh cycle is
expected to last 60 nanoseconds, the input buffers might be
de-energized for the first 40 nanoseconds. Although this approach
does reduce the power consumed during an auto-refresh cycle, it
nevertheless still allows a significant amount power to be consumed
during the period of time that the input buffers are energized. It
is generally not possible to de-energize the input buffers for
substantially the entire auto-refresh cycle because the input
buffers must be re-powered well before the end of the auto-refresh
cycle and the end of the refresh cycle cannot always be predicted
with great accuracy. Thus, de-energizing the input buffers for a
predetermined period at the start of each auto-refresh cycle still
allows the DRAM to consume a significant amount of power.
[0009] There is therefore a need for a circuit and method that
allows a more significant reduction in the power consumed by DRAMs
during an auto-refresh cycle without risk of registering spurious
commands or addresses.
SUMMARY OF THE INVENTION
[0010] A method and circuit reduces the power consumed by a dynamic
random access memory ("DRAM") during an auto-refresh. The DRAM
includes a first set of input buffers through which command signals
are coupled. The input buffers are disabled during auto-refresh so
they do not consume power responding to signals applied to their
inputs, and a plurality of command signals are biased to assert a
predetermined memory command, such as a "no operation" command.
When an internal auto-refresh timer times out, the bias is removed
from the command signals, and the input buffers are enabled. In the
event the DRAM receives a clock signal, an input buffer through
which the clock signal is coupled may also be disabled during the
auto-refresh. If so, the input buffer for the clock signal may be
re-enabled before re-enabling the input buffers for the command
signals so the timing at which the command signal input buffers are
re-enabled can be controlled relative to the clock signal. The DRAM
may also check the state of a predetermined command signal to
transition the DRAM to a low power precharge mode at the conclusion
of the auto-refresh.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a block diagram of a conventional memory device in
which the inventive power saving circuit can be used.
[0012] FIG. 2 is a block diagram of one embodiment of a power
saving circuit according to the present invention.
[0013] FIG. 3 is a timing diagram showing various signals present
in the power saving circuit of FIG. 2.
[0014] FIG. 4 is a block diagram of another embodiment of a power
saving circuit according to the present invention.
[0015] FIG. 5 is a block diagram of still another embodiment of a
power saving circuit according to the present invention.
[0016] FIG. 6 is a block diagram of a computer system using a
memory device containing a power saving circuit according to one
embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] FIG. 1 is a block diagram of a conventional synchronous
dynamic random access memory ("SDRAM") 2 that can utilize the
present invention, including one or more of the embodiments
described herein. However, it will be understood that various
embodiments of the present invention can also be used in other
types of DRAMs. The operation of the SDRAM 2 is controlled by a
command decoder 4 responsive to high level command signals received
on a control bus 6. These high level command signals, which are
typically generated by a memory controller (not shown in FIG. 1),
are a clock enable signal CKE*, a clock signal CLK, a chip select
signal CS*, a write enable signal WE*, a row address strobe signal
RAS*, a column address strobe signal CAS*, and a data mask signal
DM, in which the "*" designates the signal as active low. The
command decoder 4 generates a sequence of command signals
responsive to the high level command signals to carry out the
function (e.g., a read or a write) designated by each of the high
level command signals. These command signals, and the manner in
which they accomplish their respective functions, are conventional.
Therefore, in the interest of brevity, a further explanation of
these control signals will be omitted.
[0018] The SDRAM 2 includes an address register 12 that receives
either a row address or a column address on an address bus 14. The
address bus 14 is generally coupled to a memory controller (not
shown in FIG. 1). Typically, a row address is initially received by
the address register 12 and applied to a row address multiplexer
18. The row address multiplexer 18 couples the row address to a
number of components associated with either of two memory banks 20,
22 depending upon the state of a bank address bit forming part of
the row address. Associated with each of the memory banks 20, 22 is
a respective row address latch 26, which stores the row address,
and a row decoder 28, which decodes the row address and applies
corresponding signals to one of the arrays 20 or 22.
[0019] The row address multiplexer 18 also couples row addresses to
the row address latches 26 for the purpose of refreshing the memory
cells in the arrays 20, 22. The row addresses are generated for
refresh purposes by a refresh counter 30, which is controlled by a
refresh controller 32. The refresh controller 32 is, in turn,
controlled by the command decoder 4. More specifically, the command
decoder 4 applies either an auto-refresh command AREF or a
self-refresh command SREF command to the refresh controller 32. As
explained above, these commands cause the refresh controller to
refresh the rows of memory cells in the arrays 20, 22 in one of two
corresponding modes, namely an auto-refresh mode or a self-refresh
mode. In the auto-refresh mode, the refresh controller 32 causes
the SDRAM 2 to address each row of memory cells in the array using
the refresh counter 30 to generate the row addresses. Thus, as
mentioned above, in the auto-refresh mode, it is not necessary for
an external device to apply addresses to the address bus 14 of the
SDRAM 2. However, the auto-refresh command must be applied to the
SDRAM 2 periodically and often enough to prevent the loss of data
stored in the memory cells of the arrays 20, 22. The self-refresh
mode is essentially the same as the auto-refresh mode except that
it is not necessary to periodically apply a command to the SDRAM 2
from an external device at a rate sufficient to prevent data loss.
Instead, once the refresh controller 32 is placed in the
self-refresh mode, it automatically initiates an auto-refresh with
sufficient frequency to prevent the loss of data from the memory
cells of the arrays 20, 22.
[0020] The commands applied to the refresh controller 32 correspond
to respective combinations of the command signals applied to the
command decoder 4. These command signals are CS*, RAS*, CAS* and
WE*, and CKE. To assert either the AREF or the SREF command, CS*,
RAS*, CAS* must all be active low, and WE* must be inactive high.
The CKE signal determines whether the SDRAM 2 will cause the
command decoder to generate an auto-refresh command or a
self-refresh command. If CKE is high, the command decoder 4 will
apply an AREF command to the refresh controller 32. If CKE is low,
the command decoder 4 will apply a SREF command to the refresh
controller 32. In response to an AREF command, the SDRAM 2 will
undergo an auto-refresh cycle and will then wait for another
command, which may be another AREF command. In response to an SREF
command, the SDRAM 2 will undergo a self-refresh cycle and will
continue to do so until the CKE signal transitions high.
[0021] After the row address has been applied to the address
register 12 and stored in one of the row address latches 26, a
column address is applied to the address register 12. The address
register 12 couples the column address to a column address latch
40. Depending on the operating mode of the SDRAM 2, the column
address is either coupled through a burst counter 42 to a column
address buffer 44, or to the burst counter 42 which applies a
sequence of column addresses to the column address buffer 44
starting at the column address output by the address register 12.
In either case, the column address buffer 44 applies a column
address to a column decoder 48, which applies various column
signals to corresponding sense amplifiers and associated column
circuitry 50, 52 for one of the respective arrays 20, 22.
[0022] Data to be read from one of the arrays 20, 22 is coupled to
the column circuitry 50, 52 for one of the arrays 20, 22,
respectively. The data is then coupled to a data output register
56, which applies the data to a data bus 58. Data to be written to
one of the arrays 20, 22 are coupled from the data bus 58 through a
data input register 60 to the column circuitry 50, 52 where it is
transferred to one of the arrays 20, 22, respectively. A mask
register 64 responds to the data mask DM signal to selectively
alter the flow of data into and out of the column circuitry 50, 52,
such as by selectively masking data to be read from the arrays 20,
22.
[0023] One embodiment of a power saving circuit 100 for reducing
the power consumption of the SDRAM 2 or some other DRAM during an
auto-refresh cycle is shown in FIG. 2. Most of the power saving
circuit 100 of FIG. 2 is incorporated in the command decoder 4 of
the SDRAM 2 of FIG. 1, but a portion of the power saving circuit
100 is incorporated in the address register 12. However, it will be
understood that the power saving circuit 100 shown in FIG. 2 can be
placed in other portions of the SDRAM 2 of FIG. 1 or in other types
of memory devices.
[0024] The power saving circuit 100 includes a first set of input
buffers 102 that couple the external address bus 14 to an internal
address bus 106 to provide a plurality of internal address bits
IA.sub.0-IA.sub.N from corresponding external address bits
A.sub.0-A.sub.N. The input buffers 102 are located in the address
register 12, although, as explained above, they may also be located
elsewhere. The input buffers 102 are enabled by an active high
IBENADD signal. Similarly, a second set of input buffers 110 couple
the external control bus 6 to an internal control bus 116 to
provide a plurality of internal command signals IC.sub.0-IC.sub.N
from corresponding external command signals. These command signals
include an active low address strobe ("RAS*") signal, an active low
column address strobe ("CAS*") signal, an active low write enable
("WE*") signal, and an active low chip select ("CS*") signal. An
active high clock enable CKE signal is coupled through an input
buffer 120 to generate an internal clock enable ("ICKE") signal,
and an external clock signal is coupled through an input buffer 124
to generate an internal clock ("ICLK") signal. The input buffers
110 in the second set and the input buffer 124 for the ICLK signal
are enabled by an active high IBENCLK signal. The input buffers 110
for the command signals can be switched to a "tri-state" (i.e., a
high impedance) condition by a low command input buffer enable
IBENCMD applied to the "Z" input of the buffers 110, and to an
active low impedance state by a high IBENCMD signal.
[0025] The IBENCMD signal is coupled to the gates of several PMOS
transistors 130-134, which are coupled between a supply voltage and
respective internal command signal lines, and to the input of an
inverter 136. The inverter 136, in turn, is coupled to the gate of
an NMOS transistor 138, which is coupled between ground and the
ICS* signal line. After the input buffers 110 are enabled by a high
IBENCLK signal, the IBENCMD signal transitions high to switch the
input buffers 110 to a low impedance state and to turn OFF the
transistors 130-136 so they do not affect the operation of the
power saving circuit 100. When the input buffers 110 are switched
to a high impedance state by a low IBENCMD signal, the transistors
130-136 are turned ON to bias high respective internal command
signal lines to which they are coupled.
[0026] The internal command signals IRAS*, ICAS*, IWE*, ICS*, as
well as other internal command signals from the input buffers 110,
are applied to a command decoder unit 140. The command decoder unit
140 generates a plurality of memory commands, including an
auto-refresh command AREF, from various combinations of the command
signals applied to its inputs. As explained above, the AREF command
is asserted responsive to decoding IRAS*, ICAS*, and ICS* active
low and IWE* inactive high.
[0027] The auto-refresh command AREF is applied to a refresh
decoder 150 along with the internal clock ICLK signal and the
internal clock enable ICKE signal. Based on the state of the ICKE
signal, the refresh decoder 150 determines if the AREF command is
for an auto-refresh or if it is for a self-refresh. If ICKE is
high, the AREF command is interpreted as an auto-refresh command,
in which case the refresh decoder 150 passes the AREF command to an
output terminal as an AREF' command. If ICKE is low, the AREF
command is interpreted as a self-refresh command, in which case the
refresh decoder 150 generates a SREF command. The refresh decoder
150 command will continue to generate the SREF command until the
ICKE signal transitions high.
[0028] The AREF command is also applied to a timer 154, which
generates a Tour pulse after a predetermined period. The Tour pulse
causes the refresh decoder 150 to terminate the AREF' command,
thereby terminating the auto-refresh cycle.
[0029] All of the input buffers 110, 120, 124 as well as the
transistors 130-136, the inverter 138 the command decoder unit 140,
the refresh decoder 150 and the timer 154, are shown in FIG. 2 as
being located in the command decoder 4. However, as previously
mentioned, these components could alternatively be located
elsewhere in the SDRAM 2 or in other memory devices.
[0030] The operation of the power saving circuit 100 will now be
explained with reference to the timing diagram of FIG. 3. The
combination of control signals ("CMD") that constitute an
auto-refresh AREF command are applied to the SDRAM 2 at time To and
registered at time T.sub.1 by the rising edge of the external clock
CLK signal. The external clock enable CKE signal is high at time
T.sub.1, so the AREF command is registered as an auto-refresh
command rather than a self-refresh command. As a result, the
command decoder 140 (FIG. 2) generates a high AREF signal and the
refresh decoder 150 (FIG. 2) generates a high AREF' signal, rather
than a SREF signal, a short time after T.sub.1 to initiate the
auto-refresh cycle. The AREF command generated by the command
decoder unit 140 also triggers the timer 154, which will control
the duration of the auto-refresh cycle. In response to the
initiation of the AREF signal, the refresh decoder 150 also drives
the IBENADD, IBENCMD and IBENCLK signals low, thereby disabling the
input buffers 102, 110, 124. The input buffers 102, 110, 124 will
thereafter not respond to signal transitions applied to their
respective inputs so that they will not consume power even if the
signal transitions are rapidly occurring. As a result, the SDRAM 2
consumes relatively little power during the auto-refresh mode. The
low IBENCMD signal turns ON the transistors 130-136 thereby
maintaining the IRAS*, ICAS*, IWE* signals high and the ICS* signal
low during the auto-refresh cycle. Driving these signals in this
manner asserts a no operation ("NOP") command. However, since the
clock input buffer 124 was disabled by IBENCLK transitioning low at
time T.sub.1, the command decoder unit 140 does not register and
decode these signals as a no operation ("NOP") command.
[0031] The timer 154 generates a T.sub.OUT pulse at time T.sub.2
thereby causing the refresh decoder 150 to transition the AREF'
signal low to terminate the auto-refresh cycle. The refresh decoder
150 also drives the IBENCLK signal high at time T.sub.2 to couple
the CLK signal through the input buffer 124. If the external clock
CLK signal is low at time T.sub.2, enabling the input buffer 124
will have no effect until the next rising edge of the CLK signal.
However, if the CLK signal is high at time T.sub.2, enabling the
buffer 124 at time T.sub.2 will cause the ICLK signal at the output
of the input buffer 124 to transition at time T.sub.2, which will
register the command signals at the output of the input buffers 110
as a valid memory command. However, since the IBENCMD is still low
at time T.sub.2, the memory command is registered as a NOP command,
which will not cause the SDRAM 2 to perform any memory operation.
Significantly, the spurious rising ICLK edge will not cause the
SDRAM 2 to register a spurious command, which might occur if the
IRAS*, ICAS*, IWE*, ICS* signals were not biased to a NOP command.
The refresh decoder 150 transitions the IBENCMD signal high a
period of time after the IBENCLK signal transitions high. The high
IBENCMD signal switches the outputs of the input buffers 110 for
the command signals to a low impedance state and turns OFF the
transistors 130-136 so the IRAS*, ICAS*, IWE* signals are no longer
biased high and the ICS* signal is no longer biased low. As shown
in FIG. 3, the refresh decoder 150 also transitions the IBENADD
signal high at time T.sub.3, although it could transition the
IBENADD signal high at time T.sub.2 or some other time.
[0032] The power saving circuit 100 thus reduces the power consumed
by the SDRAM 2 during an auto-refresh cycle, and it does so in a
manner that avoids the possibility of a spurious memory command
being registered responsive to the input buffers 100 for the
command signals being enabled at the conclusion of the auto-refresh
period.
[0033] Another embodiment of a power saving circuit 200 is shown in
FIG. 4. The power saving circuit 200 is substantially identical to
the power saving circuit 100 shown in FIG. 2, and it operates in
substantially the same manner. Therefore, in the interest of
brevity, the circuit components used in the power saving circuit
200 that are identical to the circuit components used in the power
saving circuit 100 have been provided with the same reference
numerals, and an explanation of their operation will not be
repeated. The power saving circuit 200 differs from the power
saving circuit 100 by using a permanently enabled input buffer 220
to generate the internal clock ICLK signal from the external clock
CLK signal. The power saving circuit also includes an internal
clock buffer 230 that is enabled by the IBENCLK signal.
[0034] The operation of the power saving circuit 200 is
substantially the same as the power saving circuit 100.
Specifically, in response to registering an AREF command, the
IBENCMD, IBENADD and IBENCLK signals transition low to disable the
input buffers 102, 110 and the internal clock buffer 230. As a
result, neither the input buffers 102, 110 nor circuitry (not
shown) downstream from the internal clock buffer 230 consume power
during the auto-refresh cycle initiated in response to the AREF
command. However, the input buffer 220 for the clock signal and
circuitry in the refresh decoder 150 that responds to the ICLK
signal will consume power during the auto-refresh cycle. When the
timer 154 times out to generate the Tour pulse, the refresh decoder
150 can simply wait for half the period of the ICLK signal after
the preceding rising edge of the ICLK signal to transition the
IBENCMD, IBENADD and IBENCLK signals high. The power saving circuit
200 thus has the disadvantage of consuming more power than the
power saving circuit 100 of FIG. 2, but it has the advantage of
being able to enable the input buffers 102, 110 without generating
a spurious command.
[0035] Another embodiment of a power saving circuit 300 is shown in
FIG. 5. The power saving circuit 300 is also very similar to the
power saving circuit 100 shown in FIG. 2, and it initially operates
in substantially the same manner. Therefore, in the interest of
brevity, the circuit components used in the power saving circuit
300 that are identical to the circuit components used in the power
saving circuit 100 have been provided with the same reference
numerals, and an explanation of their operation will not be
repeated. The power saving circuit 300 differs from the power
saving circuit 100 by allowing the SDRAM 2 to operate in a mode
that automatically transitions the SDRAM 2 to a power saving
precharge mode at the conclusion of a reduced power auto-refresh
cycle. In addition to the components used in the power saving
circuit 100 of FIG. 2, the power saving circuit of FIG. 5 includes
a mode decoder 310 that decodes the CKE signal and a data mask
("DM") signal applied to a DM input terminal. As explained above,
the DM signal is used to mask data being read from or written to
the SDRAM 2. Thus, the DM terminal is not needed during a refresh
of the SDRAM 2 because data are not being read from or written to
the SDRAM 2. Although the DM input terminal is used in the
embodiment shown in FIG. 5, it will be understood that some other
terminal that is not used during refresh may be used to assert an
auto-refresh command.
[0036] The mode decoder decodes these signals as follows:
1 MODE DM CKE Low Power AREF Mode With "0" "0" (for full AREF
period) Low Power Precharge Low Power AREF Mode "0" "1" Without Low
Power Precharge Normal AREF Mode "1" "0" Normal SREF Mode "1"
"1"
[0037] Thus, if the DM signal is high when the AREF or SREF
commands are asserted, the SDRAM 2 operates in a conventional
manner. However, if the DM signal is low when the AREF command is
asserted, the SDRAM 2 operates in the low power AREF mode described
above with reference to FIGS. 2 and 3 regardless of the state of
the CKE signal. If the CKE signal is high when the AREF command or
at any time during the auto-refresh, when the T.sub.OUT pulse is
generated to end of the AREF cycle, the SDRAM 2 returns to its
normal operating mode to wait for another memory command. However,
if the CKE signal is low when the AREF command is asserted and
remains low during the entire auto-refresh cycle, the refresh
decoder 150' generates an active high low power precharge ("LPP")
signal when the T.sub.OUT pulse is generated to end of the AREF
cycle. Also, in the low power precharge mode, the SDRAM 2 remains
in the low power AREF mode so that the refresh decoder 150' does
not transition the IBENCMD, IBENADD and IBENCLK signals high at the
end of the AREF cycle. Circuitry in the SDRAM 2 (not shown)
responds to the high LPP signal to remove power from circuit
components in the SDRAM 2 that need not be powered to retain data
stored in the memory arrays 20, 22 (FIG. 1). For example, power may
be removed from the command decoder 4 (FIG. 1), the column decoder
48, and some of the row decoders 28.
[0038] The SDRAM 2 remains in the low power AREF mode as described
above and in the low power precharge mode until the CKE signal
transitions high. Also, as mentioned previously, if the CKE signal
transitions high at any time during the AREF cycle, the active high
LPP signal will not be generated at the end of the AREF cycle. When
the CKE signal transitions high, the refresh decoder 150'
transitions the IBENCMD, IBENADD and IBENCMD signals active high as
described above. The refresh decoder 150' also transitions the LPP
signal inactive low to re-apply power to circuitry in the SDRAM 2.
The low power AREF mode with the LPP mode thus not only minimizes
the power consumed by the SDRAM 2 during an auto-refresh cycle, but
it also automatically switches the SDRAM 2 to an operating mode at
the end of the auto-refresh cycle in which even less power is
consumed.
[0039] Although the power saving circuit 300 shown in FIG. 5 uses
the DM signal to differentiate between low power auto-refresh modes
with and without the low power precharge mode, other means of
differentiating between these modes can be used. For example, a
conventional mode register (not shown) could be programmed with one
or more bits during initialization of the SDRAM 2 to indicate a
selected operating mode.
[0040] FIG. 6 shows an embodiment of a computer system 400 that may
use the SDRAM 2 or some other memory device that contains an
embodiment of a power saving circuit as described herein or some
other embodiment of a power saving circuit in accordance with the
invention. The computer system 400 includes a processor 402 for
performing various computing functions, such as executing specific
software to perform specific calculations or tasks. The processor
402 includes a processor bus 404 that normally includes an address
bus, a control bus, and a data bus. In addition, the computer
system 400 includes one or more input devices 414, such as a
keyboard or a mouse, coupled to the processor 402 to allow an
operator to interface with the computer system 400. Typically, the
computer system 400 also includes one or more output devices 416
coupled to the processor 402, such output devices typically being a
printer or a video terminal. One or more data storage devices 418
are also typically coupled to the processor 402 to store data or
retrieve data from external storage media (not shown). Examples of
typical storage devices 418 include hard and floppy disks, tape
cassettes, and compact disk read-only memories (CD-ROMs). The
processor 402 is also typically coupled to a cache memory 426,
which is usually static random access memory ("SRAM") and to the
SDRAM 2 through a memory controller 430. The memory controller 430
includes an address bus coupled to the address bus 14 (FIG. 1) to
couple row addresses and column addresses to the DRAM 2, as
previously explained. The memory controller 430 also includes a
control bus that couples command signals to a control bus 6 of the
SDRAM 2. The external data bus 58 of the SDRAM 2 is coupled to the
data bus of the processor 402, either directly or through the
memory controller 430. The memory controller 430 applies
appropriate command signals to the SDRAM 2 to cause the SDRAM 2 to
operate in one or more of the power saving modes described
above.
[0041] From the foregoing it will be appreciated that, although
specific embodiments of the invention have been described herein
for purposes of illustration, various modifications may be made
without deviating from the spirit and scope of the invention.
Accordingly, the invention is not limited except as by the appended
claims.
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